Hello friends!
Yes, Mimosa is orally active by itself in much greater quantities, however, I've never heard a particular reason purposed such as GOD's suggestion that it's due to carbolines.
I tried this out once trying (once again) to prove Mr. McKenna wrong. From a batch of mimosa that would have done the trick with an maoi at 15 grams, it took 35g of mimosa to produce a true, dmt trip. This was also done through cold water extraction; just letting the powdered mimosa sit in cold water for 2 days, shaking occasionally.